German Patent Publication No. DE 195 14 572 discloses a supercharged internal combustion engine. The supercharged internal combustion engine encompasses a high-pressure section and a low-pressure section that is larger as compared with the high-pressure section, in which engine, using a switchover apparatus, the high- and low-pressure sections are connected in series in the lower rotation-speed range of the internal combustion engine, and as the rotation speed of the internal combustion engine rises, the exhaust gas is for the most part guided, by switchover of the switchover apparatus, past a high-pressure turbine and introduced directly into a low-pressure turbine. The low-pressure turbine is embodied in double-flow fashion; in two-stage operation, switchover apparatus 13 connects the output of the high-pressure turbine via exhaust-gas ducts to both bores of the low-pressure turbine, and that after rotation of a flap valve of the switchover apparatus, the high-pressure turbine is bypassed and separate exhaust sections of the internal combustion engine are connected directly, using the exhaust-gas ducts, to the two bores of the low-pressure turbine. A venting valve is provided that is inserted into a bypass duct, which duct connects the exhaust section to a connector fitting of the switchover apparatus; and that the venting valve is opened upon a smooth transition from two-stage to single-stage operation, so that in the transition region between the lower and medium rotation speed of the internal combustion engine, exhaust gas can partially be delivered out of the exhaust section directly to the low-pressure turbine. In single-stage operation, the high-pressure compressor can be bypassed by way of a boost air duct connected in parallel, so that only a small volume of cool air flows through the high-pressure compressor. This system is made up in principle of two exhaust-gas turbochargers connected one behind the other: the high-pressure and low-pressure exhaust-gas turbochargers. The air mass that is delivered can be greatly increased by the two-stage supercharging, and emissions and/or engine power output can thus be improved. Two-stage supercharging is utilized in large-engine design, e.g. in ship drive systems or in power-generating stations. Additionally known are mechanical supercharging systems in which the compressor is driven via a transmission directly by the internal combustion engine. The drive power required for the compressor is obtained not from the exhaust-gas energy, but instead is made available directly at the internal combustion engine.
In commercial-vehicle engines, turbocompound systems are by now preferably also being used. In these systems, the usable output of the internal combustion engine is generated not by the working cylinder but also with a supercharging device such as, for example, an expansion section downstream from an exhaust-gas turbocharger. This additionally generated power is fed back to the crankshaft of the internal combustion engine via a reduction transmission, a freewheel, and a damping clutch, or alternatively via a hydraulic converter. This system is used in commercial vehicles in order to make further use of the energy remaining in the exhaust gas after the turbocharger has been driven, and thus further to reduce fuel consumption.
The aforementioned supercharging devices for internal combustion engines present certain problems, however. For example, a rise in boost pressure can be achieved only by increasing the charger rotation speed, for which on the one hand sufficient exhaust-gas energy must be present, and the latter must also be sufficient additionally to overcome the moment of inertia of the charger as it is accelerated to higher rotation speeds. The dynamic behavior of such a system is thus subject to limitations.
The aforementioned turbocompound system is used for optimum utilization of exhaust-gas energy, but can feed energy back to the crankshaft of the internal combustion engine in satisfactory fashion only at higher loads and rotation speeds of the internal combustion engine.
Performance losses occur in the context of exhaust-gas recirculation applications and supercharging systems used in standard fashion hitherto. Emissions adaptation at high exhaust-gas recirculation rates is possible, in supercharging systems used at present, only at the cost of a decrease in rated power, since a supercharging system complying with Euro 4 commercial-vehicles standards requires high exhaust-gas recirculation rates even with the internal combustion engine at full load. In conventional supercharging systems, however, particulate emissions exceed the limit value in order to maintain NOx emissions, and thus require the use of an exhaust post-treatment system, e.g. a diesel particulate filter.